Electromagnetic survey system with multiple sources

ABSTRACT

Multiple sources are provided for a transmitter cable for use in electromagnetic surveying. The transmitter cable includes a dipole antenna comprising a pair of spaced apart electrodes mounted on their respective cables. Two antennas may be powered from each source. Alternatively, the outputs of each source are connected to a common antenna pair. A single large power supply may be mounted on a vessel to supply power through the tow cable. Alternatively, a number of power supplies may be provided aboard the vessel, with each power supply having dedicated conductors through the tow cable to power the plurality of current sources.

FIELD OF THE INVENTION

The present invention relates generally to the field of electromagneticsurvey systems used in sub-sea exploration for hydrocarbons and, moreparticularly, to such a system having a plurality of sources.

BACKGROUND OF THE INVENTION

Various systems have been deployed to determine the response of theearth's sub-surface strata to electromagnetic fields for geophysicalresearch. Electromagnetic (EM) surveying, or sounding, techniques canprovide valuable insights into the likely hydrocarbon content ofsubterranean reservoirs. Lately, EM surveying systems and techniqueshave been receiving increasing interest in commercial applications inthe search for oil and gas.

Seismic techniques are often used during oil exploration to identify theexistence, location, and extent of reservoirs in subterranean rockstrata. During seismic exploration, a sound signal is transmitted to thesub-surface strata where the signal encounters geologic anomalies. Theseismic signal is then reflected back to receivers such as hydrophonesfor sub-sea exploration. The signals thus received are analyzed for theappearance of the sub-sea structures, ideally indicative of the presenceof hydrocarbons.

Although seismic surveying is able to identify such structures, thetechnique is often unable to distinguish between the different possiblecompositions of pore fluids within them, especially for pore fluidswhich have similar mechanical properties. In the field of oilexploration, it is necessary to determine whether a previouslyidentified reservoir contains oil or just aqueous pore fluids. To dothis, often an exploratory well is drilled to determine the contents ofthe reservoir. However, this is an expensive process, and one whichprovides no guarantee of reward.

Thus, while oil-filled and water-filled reservoirs are mechanicallysimilar, they do possess significantly different electrical propertiesand these properties provide for the possibility of electromagneticbased discrimination testing. Also, seismic techniques are not welladapted to the detection of certain other resistivity contrasts whichmay be useful in the identification of likely candidates for furtherhydrocarbon exploration.

A known technique for electromagnetic probing of subterranean rockstrata is the passive magneto-telluric (MT) method, as described inGB2390904 to University of Southampton. In such a method, the signalmeasured by a surface-based electromagnetic detector in response toelectromagnetic (EM) fields generated naturally, such as within theearth's upper atmosphere, can provide details about the surroundingsubterranean rock strata.

However. for deep-sea surveys, all but those MT signals with periodscorresponding to several cycles per hour are screened from the sea floorby the highly conductive seawater. Although long wavelength signalswhich do penetrate to the sea floor can be used for large scale underseaprobing, they do not provide sufficient spatial resolution to examinethe electrical properties of the typically relatively small scalesubterranean reservoirs. Moreover, since MT surveying relies primarilyon horizontally polarized EM fields, it is intrinsically insensitive tothin resistive layers.

Nonetheless, measurements of electrical resistivity beneath the seafloor have traditionally played a crucial role in hydrocarbonexploration and reservoir assessment and development. There are clearadvantages to developing non-invasive geophysical methods capable ofproviding such information from the surface or seafloor. For example,the vast savings that may be realized in terms of avoiding the costs ofdrilling test wells into structures that do not contain economicallyrecoverable amounts of hydrocarbon would represent a major economicadvantage.

In research fields that are not of commercial interest, geophysicalmethods for mapping subterranean resistivity variations by various formsof EM surveying have been in use for many years. Proposals for findinghydrocarbon reservoirs using such EM surveying have also been made andapplications to the direct detection of hydrocarbons using horizontalelectric dipole (HED) sources and detectors have proved successful.

Thus, CSEM (Controlled Source Electromagnetics), is a technique oftransmitting discrete, very low frequency electromagnetic energy using acurrent generating source, and an electric dipole. CSEM maps theresistivity contrasts in the subsurface. The method is sensitive torelatively high resistive formations imbedded in a relatively lowresistive formation. The frequency range in CSEM is typically between1/32 Hz to 32 Hz; however, most applications are sub-hertz (less than 1Hz).

In operation, a fleet of ocean-bottom receivers are deployed. Thetransmitting source is then towed above these receivers, and thereceivers detect the transmitted EM field, which is altered by thepresence of the varying resistivity of the subsurface within the rangeof the receivers.

A typical CSEM surveying system would have a transmitter antenna for usein EM surveying beneath the ocean floor and would include a currentsource housed in a fish and a dipole antenna. The dipole antennacomprises a first electrode mounted on a cable and located near thecurrent source and a second electrode mounted on a cable and locatedfarther away from the current source. Each electrode is electricallyconnected to the current source. The transmitter antenna may be deployedby being towed behind a vessel and various sensors may be mounted oneach cable.

While such systems show promise in commercial exploration, they sufferfrom certain drawbacks and limitations. The range of such systems isdetermined by many factors, including frequency and current of thecurrent source. Such systems have a single source, and thus thefrequency and current are established by engineering factors, and cannotbe varied outside rather limited parameters. The current state of theart involves using a single current source to output up to about 1600Amperes. Those of skill in this art generally consider approximately1700 Amperes to be the physical limit for a single source application,limited primarily by the cross-sectional diameter of current carryingconductors and the state of the art in the effectiveness of insulators.

Furthermore, such systems have a limited number or even just a singletransmitter geometry, and are not adapted to scaling by the addition ofcomponents in various arrangements.

Thus, there remains a need for an EM system which provides flexibilityin the arrangement of plural power sources to increase the effectiverange and capability of such known systems. The present inventionaddresses these and other drawbacks of the art.

SUMMARY OF THE INVENTION

The present invention solves these noted problems in the art byproviding multiple sources in a controlled source electromagnetic (CSEM)survey system. In a first aspect, the present invention comprises atransmitter cable for use in electromagnetic surveying. The transmittercable includes a dipole antenna comprising a first electrode on anantenna cable of a first length and a second electrode on an antennacable of a second length; and a plurality of current sourceselectrically coupled to the dipole antenna.

In another aspect of the invention, a plurality of sources for atransmitter cable are provided. In a presently preferred embodiment, twoantennas may be powered from each source. Alternatively, the outputs ofeach source are connected to a common antenna pair.

Also, a single large power supply may be mounted on a vessel to supplypower through the tow cable. Alternatively, a number of power suppliesmay be provided, with each power supply having dedicated conductorsthrough the tow cable to power the plurality of current sources.

These and other aspect, objects, and advantages of the invention will bereadily apparent to those of skill in the art from a review of thefollowing detailed description along with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects of the present invention are attained and can be understood indetail, more particular description of the invention, briefly summarizedabove, may be had by reference to embodiments thereof which areillustrated in the appended drawings.

FIG. 1 is a schematic diagram of a known, prior art CSEM survey system.

FIG. 2 is a schematic diagram of a CSEM survey system of the presentinvention using a plurality of sources, with three of such sourcesillustrated.

FIG. 3 is an electrical schematic diagram showing the distribution ofpower to a plurality of sources from a single source of electrical poweraboard a vessel.

FIG. 4 is an electrical schematic diagram showing power distributionfrom a pair of sources to a single, dipole antenna.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a typical single source EM surveying systemtransmitter antenna 10 towed behind a vessel 12 by a tow cable 14. Theantenna 10 comprises a unitary current source 16 which may be housedwithin a fish for buoyancy to maintain a predetermined altitude abovethe ocean floor. The current source 16 provides current to a dipoleantenna 18 which comprises a first electrode 20 mounted on a first,relatively short cable 22 and a second electrode 24 mounted on a second,relatively long cable 26. In that way, the first electrode is locatednearer the source and the second electrode is located farther away fromthe source and the electromagnetic energy flows between the twoelectrodes through the seawater between them.

Both electrodes are powered from the same unitary source 16, and aretherefore limited to the current provided by that one source and at thesame frequency. The present invention is directed to overcoming thislimitation in the art.

FIG. 2 depicts a transmitter antenna 30 of the present invention towedbehind the vessel 12 by the tow cable 14, as before. In this instance,however, the transmitter antenna 30 comprises a dipole antenna 34 havingtwo or more sources, shown as sources 32, 32′, and 32″ in FIG. 2,connected in parallel, although other geometries and antenna types maybe used. The sources 32, 32′, and 32″ provide a plurality of currentsand frequencies to the electrodes 20 and 24 through their respectivecables 22 and 26.

The present invention has been described as comprising two or moresources. However, more than four such sources can become cumbersome toconstruct together, and may not provide much additional technicalcapability in relation to the increased complexity. However, theinvention contemplates and includes such arrangements though they maynot be preferred embodiments.

A number of arrangements may be used in powering a plurality of currentsources from the vessel. One simple arrangement comprises providing asingle, large power supply unit (PSU) aboard the vessel and conductingthe power from the vessel to the plural sources through the tow cable.The tow cable may include one or more power conductors, and the powerthus provided is common to each source.

FIG. 3 depicts an electrical schematic for supplying a plurality ofsources from a single PSU aboard the vessel. In this arrangement, poweris conducted by conductors within the tow cable 14 to a junction orsplitter box 40. The junction box 40 provides power to the sourceslabeled 42, 42′, and 42″ in FIG. 3. The power is carried over powercables 44, 44′, and 44″, respectively and is provided to the junctionbox from a power supply 46 aboard the vessel (see FIG. 2). Note that,with the arrangement as shown in FIG. 3, using multiple sources inparallel, a sum of the output currents is provided to the electrodes.

In an alternative preferred embodiment, a plurality of power supplyunits are provided aboard the vessel. By having a plurality of PSU's,one such PSU may be dedicated to providing power to each sourceseparately. The tow-cable in that case comprises multiple powerconductors. A pair of conductors (power and return) is dedicated to eachsource, i.e. two sources require a quad cable, and so on. One advantageof this scheme is that each source is isolated from the other (via theseparate PSUs) and thus the power and frequency spectrum may be tailoredto each environment for the survey. For example, a larger number offrequencies can be generated than may be possible with a complexwaveform intended to deconvolve into multiple frequencies. Also, thepower or current delivered at each frequency can be individuallycontrolled, unlike the constraints inherent in a single waveform inwhich power delivered often declines with increased frequency.Furthermore, synchronizing each source to the other is a trivial matteras the AC output from the PSU (or multiple PSU's) to each source can beGPS locked. Thus, the invention allows an operator to use specificfrequencies designed to reach different targets at different depths, andthereby provides the operator great flexibility in designing the survey.

Several schemes are possible for supplying current from each source tothe antennas. One scheme involves keeping the antennas separate for eachsource, i.e. maintaining two antennas per source. This scheme is thesimplest to implement electrically. By keeping the antennas andelectrodes in parallel, each source sees its output impedance only; thatis, adding multiple sources does not change the output impedance ofanother source. However, from an operational point of view, this schemeis more difficult than others.

In an alternative scheme, the outputs of each source are connected to acommon antenna pair. In this scheme, each source must be electricallybuffered or isolated from the others. This is achieved, for example,using power diodes on each output, as shown in FIG. 4. FIG. 4 shows afirst source 50 and a second source 50′. The first source 50 includes apositive output 52 and a negative output 54 and the second source 50′includes a positive output 52′ and a negative output 54′. The positiveoutputs 52, 52′ are coupled to the cable 26 and thence to the electrode24, and similarly the negative outputs 54, 54′ are coupled to the cable22 and thence to the electrode 20. The positive and negative outputscould also be coupled reciprocally to electrodes 20, 24, respectively.The power diodes thereby isolate the sources 50 and 50′ from each other.

Operationally this is straightforward to deploy. However, the ability todeliver current becomes very dependent on the antenna impedance andcontact impedance, i.e. the impedance of one antenna pair sets the totalload impedance.

In summary, using multiple sources for the controlled sourceelectromagnetic survey system as just described whose outputs arefrequency and phase locked results in a total output equal to thesummation of the individual sources. For example, using three 800 Asources is equivalent to a single 2400 A source. Also, using threesources allows the partitioning of the output energy in the frequencydomain. In other words, if three sources are used, the first source maybe directed to output lower frequencies, while the other two sourcesoutput medium and higher frequencies, respectively. Finally, a multiplesource system may be used to partition energy into X, Y, and Zcomponents. Currently, only the X axis is used in a typical system, asillustrated in FIG. 1. The system of the present invention as describedin detail is inherently scaleable and configurable in power output,frequency distribution, and transmitter geometry.

The principles, preferred embodiment, and mode of operation of thepresent invention have been described in the foregoing specification.This invention is not to be construed as limited to the particular formsdisclosed, since these are regarded as illustrative rather thanrestrictive. Moreover, variations and changes may be made by thoseskilled in the art without departing from the scope of the invention.

1. A transmitter cable for use in electromagnetic surveying, comprising:an antenna; and a plurality of current sources electrically coupled tothe antenna.
 2. The transmitter cable of claim 1, wherein the antenna isa dipole antenna.
 3. The transmitter cable of claim 1, furthercomprising: a first electrode on an antenna cable of a first length; anda second electrode on an antenna cable of a second length.
 4. Thetransmitter cable of claim 3, wherein each of the plurality of currentsources is electrically coupled to the first length cable and the secondlength cable.
 5. The transmitter cable of claim 4, further comprising apower isolation diode between each of the plurality of current sourcesand the first length cable and the second length cable.
 6. Thetransmitter cable of claim 1, wherein the plurality of current sourcesare electrically coupled in parallel.
 7. The transmitter cable of claim1, further comprising a common source of electrical power to theplurality of current sources.
 8. The transmitter cable of claim 1,further comprising a separate source of electrical power electricallycoupled to each of the plurality of current sources.
 9. The transmittercable of claim 1, wherein at least two of the plurality of currentsources operate at different frequencies.
 10. The transmitter cable ofclaim 1, wherein at least two of the plurality of current sourcesoperate at different currents.
 11. The transmitter cable of claim 1,wherein at least two of the plurality of current sources operate atdifferent frequencies and different currents, and the current deliveredat each frequency is selectively controlled.
 12. An electromagneticsurvey system comprising: a transmitter cable comprising: an antenna;and a plurality of current sources electrically coupled to the antenna;a source of electrical power for the current sources; a plurality ofreceivers; and means for towing the transmitter cable through waterproximate the plurality of receivers.
 13. The system of claim 12,wherein the means for towing includes a tow cable coupled to a vessel.14. The system of claim 13, further comprising: an electrical conductorthrough the tow cable; and a junction box between the electricalconductor and the plurality of current sources.
 15. The system of claim13, further comprising a plurality of electrical conductors through thetow cable, the plurality of electrical conductors comprising anelectrical pair of conductors for each of the plurality of currentsources.
 16. The system of claim 12, wherein the transmitter cablefurther comprises: a first electrode on an antenna cable of a firstlength; and a second electrode on an antenna cable of a second length.17. The system of claim 16, wherein each of the plurality of currentsources is electrically coupled to the first length cable and the secondlength cable.
 18. The system of claim 17, further comprising a powerisolation diode between each of the plurality of current sources and thefirst length cable and the second length cable.
 19. The system of claim12, wherein the plurality of current sources are electrically coupled inparallel.
 20. The system of claim 12, wherein at least two of theplurality of current sources operate at different frequencies.
 21. Thesystem of claim 12, wherein the antenna is a dipole antenna.
 22. Thetransmitter cable of claim 12, wherein at least two of the plurality ofcurrent sources operate at different frequencies and different currents,and the current delivered at each frequency is selectively controlled.